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Date: Thu, 8 Apr 2021 16:45:37 +0000 From: Al Viro <viro@...iv.linux.org.uk> To: Daniel Xu <dxu@...uu.xyz> Cc: bpf@...r.kernel.org, linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org, kernel-team@...com, jolsa@...nel.org, hannes@...xchg.org, yhs@...com Subject: Re: [RFC bpf-next 1/1] bpf: Introduce iter_pagecache On Wed, Apr 07, 2021 at 02:46:11PM -0700, Daniel Xu wrote: > +static void fini_seq_pagecache(void *priv_data) > +{ > + struct bpf_iter_seq_pagecache_info *info = priv_data; > + struct radix_tree_iter iter; > + struct super_block *sb; > + void **slot; > + > + radix_tree_for_each_slot(slot, &info->superblocks, &iter, 0) { > + sb = (struct super_block *)iter.index; > + atomic_dec(&sb->s_active); > + radix_tree_delete(&info->superblocks, iter.index); > + } ... and if in the meanwhile all other contributors to ->s_active have gone away, that will result in...? IOW, NAK. The objects you are playing with have non-trivial lifecycle and poking into the guts of data structures without bothering to understand it is not a good idea. Rule of the thumb: if your code ends up using fields that are otherwise handled by a small part of codebase, the odds are that you need to be bloody careful. In particular, ->ns_lock has 3 users - all in fs/namespace.c. ->list/->mnt_list: all users in fs/namespace.c and fs/pnode.c. ->s_active: majority in fs/super.c, with several outliers in filesystems and safety of those is not trivial. Any time you see that kind of pattern, you are risking to reprise a scene from The Modern Times - the one with Charlie taking a trip through the guts of machinery.
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